Samsung has been developing their own ecosystem for a while now -- it's a way to try to get people to keep purchasing Samsung devices. For a while, there were even rumors that Samsung wanted to abandon Android and just go with their own OS (Tizen), but the few Tizen phones that came out were pretty much abysmal failures, so they ditched that idea.
You can always install Google's version of the various apps (like Contacts, Calendar, Gmail, etc.). Since those Google apps are universally available across all Android devices, that's the best way to hedge your bets, in case you ever decide to step away from Samsung. But you're right, if you get deeply involved in Samsung's ecosystem, it can be a pain to move to another manufacturer and keep all of that data.
I found the data transfer to be ok.
It's just that the simplicity and UI of Phone, Messages, and Visual Voicemail were better than I realized, plus OneTouch.
Phone: I can view log history in one click, unlike Google Phone, which takes two. Plus Phone also shows me SMS history when I click on that contact. It will show me phone or SMS history.
Messages: It's just a cleaner layout.
Visual Voicemail: Well if I don't want Google Phone because of the aforementioned two click history reason, I have to search for a new voicemail. But most of those other apps either have ads, or they don't work. Google Phone had a voicemail app within it. But I couldn't find a 3rd party voicemail that I liked or that worked, that didn't have ads.
OneTouch: I've been having problems with Xperia's launcher not showing a new message counter when using Next SMS by Handcent. I have a support thread with them. One Touch on Samsung never had a problem like that.
I also tried Nova Launcher and S-Launcher, so it may be a problem with Next SMS specifically. But still, just the fact that stuff with Samsung's launcher and apps worked, and were easier, makes me miss that.
BTW I created a few threads here, in case anyone wanted to read or comment on them.
https://forums.androidcentral.com/sony-xperia-5/
Probably the most seamless thing has been the side fingerprint on the Xperia. It's been as easy to use as Samsung's front fingerprint on the Note 4 was.